Femtosecond laser processing system with process parameters, controls and feedback

ABSTRACT

A femtosecond laser based laser processing system having a femtosecond laser, frequency conversion optics, beam manipulation optics, target motion control, processing chamber, diagnostic systems and system control modules. The femtosecond laser based laser processing system allows for the utilization of the unique heat control in micromachining, and the system has greater output beam stability, continuously variable repetition rate and unique temporal beam shaping capabilities.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 10/813,269filed Mar. 31, 2004, which is the disclosure of which is incorporatedherein by reference in its entirety. This application is related to thefollowing co-pending and commonly-assigned patent applications:

U.S. application Ser. No. 10/813,163, attorney's docket number A8698,entitled “MODULAR FIBER-BASED CHIRPED PULSE AMPLIFICATION SYSTEM,” filedon same date herewith, the disclosure of which is hereby incorporated byreference.

U.S. application Ser. No. 10/813,173, attorney's docket number A8699,entitled “METHOD AND APPARATUS FOR CONTROLLING AND PROTECTING PULSEDHIGH POWER FIBER AMPLIFIER SYSTEMS,” filed on same date herewith, thedisclosure of which is hereby incorporated by reference.

U.S. application Ser. No. 10/813,389, attorney's docket number A8701,entitled “PULSED LASER PROCESSING WITH CONTROLLED THERMAL AND PHYSICALALTERATIONS,” filed on same date herewith, the disclosure of which ishereby incorporated by reference.

U.S. application Ser. No. 10/813,161, attorney's docket number A8732,entitled “ETCHED PLATE ALIGNMENT METHOD,” filed on same date herewith,the disclosure of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present invention is directed to a laser processing apparatus thatuses ultrashort laser pulses for materials processing with micron-levelprecision. The laser processing system allows for active control andoptimization of the laser/material interaction.

2. Description of the Related Art

Femtosecond lasers offer several unique advantages over lasers of longerpulse duration. In particular, their ultrashort pulse duration makes itpossible to produce extremely high target intensities with relativelylow pulse energy. The high target intensities, in conjunction withultrashort pulse duration, enable precise micron-level materialsprocessing with minimal and/or manageable heat transfer to the targetsubstrate per pulse. It is possible to take unique advantage of thislatter property by controlling the rate of laser impact upon the targetsubstrate. However, for optimal and practical application of the uniqueproperties of femtosecond lasers, a laser processing system is required,which integrates and coordinates the following: laser operations, beammanipulation, target positioning and processing environment. The laserprocessing system must also provide real-time process monitoring. Thisintegration is very crucial to achieve the best possible processingresults for a given application that uses the laser processing system.Also, from a practical standpoint, a well controlled, modular andflexible laser processing system is crucial to process a variety ofmaterials.

SUMMARY OF THE INVENTION

This invention allows for precise control over laser materialsprocessing by integrating a femtosecond laser, beam manipulation optics,target control and diagnostics into a system whereby the operation ofthe subcomponents can be individually or cooperatively changed. As such,the system allows for “on the fly” variation of a wide variety ofprocessing parameters. Thus it is possible to tailor the systemoperation for a particular application and verify that the desiredresult is being achieved. Additional aspects and advantages of theinvention will be set forth in part in the description that follows andin part will be obvious from the description, or may be learned bypractice of the invention. The aspects and advantages of the inventionmay be realized and attained by means of the instrumentalities andcombinations particularly pointed out in the appended claims.

An aspect of the present invention is an integrated femtosecond laserbased laser processing system that comprises a femtosecond laser,frequency conversion optics, beam manipulation optics, target motioncontrol, processing chamber, diagnostic systems and system controlmodules. In order to demonstrate the unique capabilities of such anintegrated system, several applications enabled by such a laserprocessing system are described as well.

In another aspect of the present invention, an apparatus for generatingoptical pulses, wherein each pulse may have individualizedcharacteristics, is provided. The apparatus comprises a laser means forgenerating the bursts of pulses, a control means that controls the lasermeans and a beam manipulation means for monitoring the pulse width,wavelength, repetition rate, polarization and/or temporal delaycharacteristics of the pulses comprising the pulse bursts. The apparatusgenerates feedback data based on the measured pulse width, wavelength,repetition rate, polarization and/or temporal delay characteristics forthe control means. In one embodiment of the present invention, the lasermeans may comprise a fiber amplifier that uses stretcher gratings andcompressor gratings. The beam manipulation means can comprise a varietyof devices, e.g., an optical gating device that measures the pulseduration of the laser pulses, a power meter that measures the power ofthe laser pulses output from the laser means or a photodiode thatmeasures a repetition rate of the laser pulses. Another beammanipulation means optically converts the fundamental frequency of apercentage of the generated laser pulses to one or more other opticalfrequencies, and includes at least one optical member that converts aportion of the fundamental of the laser pulses into at least one higherorder harmonic signal. The optical member device may comprise anon-linear crystal device with a controller that controls the crystal'sorientation. Preferably, the means for converting an optical frequencyincludes a spectrometer that measures predetermined parameters of pulsesoutput from the non-linear crystal device and generates feedback for thecontrol means. Another embodiment of the beam manipulation meanscomprises telescopic optical devices to control the size, shape,divergence or polarization of the laser pulses input, and steeringoptics to control an impingement location of the laser pulses on atarget substrate. The apparatus may further comprise a beam profilerthat monitors characteristics of laser pulses and generates feedback forthe control means. The above-described apparatus has several end uses,such as modifying the refractive index of a target substrate; surfacemarking, sub-surface marking and surface texturing of a targetsubstrate; fabricating holes, channels or vias in a target substrate;and depositing or removing of thin layers of material on a targetsubstrate.

In another aspect of the present invention, an apparatus for machining atarget substrate using ultrafast laser pulses is provided. The apparatuscomprises a laser device that generates ultrafast laser pulses to beused in a machining process. Preferably, the laser device comprises afemtosecond fiber laser with variable output parameters and acontroller, which provides for active change/adjustment of the outputparameters. The laser device may also incorporate additional devices tomeasure the output beam characteristics for control purposes. Theapparatus further comprises an optical frequency converter tofrequency-convert the generated ultrafast laser pulses. The opticalfrequency converter can include a non-linear optical crystal forperforming the frequency conversion. The optical frequency converter canfurther include a telescope to focus the input ultrafast pulses throughthe non-linear crystal and to collimate the pulses output from thenon-linear crystal. The optical frequency converter can (but need not)also include optical members to separate the converted frequencies fromthe harmonic beam, such that it is possible to control which opticalfrequency component and/or combination of frequencies impinge upon thetarget. The apparatus further includes a beam manipulating device thatalters the physical characteristics of the ultrafast laser pulses, aswell as controls the impingement location of the pulses with respect tothe target substrate. The beam manipulating device includes variousoptical devices for controlling the size, shape, divergence andpolarization of the ultrafast laser pulses. The beam manipulating devicecan also (but need not) include a set of active steering optics todirect where the manipulated beam impinges upon the target substrate.The apparatus further includes a focusing means comprised of optics thatconcentrates the ultrafast laser pulses onto the desired locations ofthe target substrate. The apparatus further comprises a targetmanipulation device for positioning the target substrate, whichpositions and moves the target substrate with respect to the laserpulses output from the focusing apparatus. The target manipulationdevice can also maintain the temperature of the target substrate asrequired by the particular process being executed. The targetmanipulation device can be enclosed in an environmental chamber if theparticular processing application requires a controlled environment orthe introduction of gasses at certain temperatures and/or pressures. Theapparatus also includes diagnostics, which monitor the laser/materialinteraction and confirm the performance of the laser device, the opticalfrequency converter, the beam manipulation device, the focusing means,and the target manipulation device. The apparatus also includes acomputer that executes software programs and is coupled to theindividual components of the system. The computer executes a program(s)that coordinates the action of the laser device, the optical frequencyconverter, the beam manipulation device, the focusing means, and thetarget manipulation device. The computer receives feedback from theindividual components and diagnostics in order to control the particularprocess being applied to a target substrate.

The above and other aspects and advantages of the invention will becomeapparent from the following detailed description and with reference tothe accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification illustrate embodiments of the invention and,together with the description, serve to explain the aspects, advantagesand principles of the invention. In the drawings,

FIG. 1A illustrates a burst comprised of two laser light pulses that areseparated in time.

FIG. 1B illustrates a burst comprised of two laser light pulses that areoverlapped.

FIG. 2A illustrates burst comprised of two laser light pulses, withdifferent wavelengths, that are not overlapped in time.

FIG. 2B illustrates a burst comprised of two laser light pulses, withdifferent wavelengths, that are overlapped in time.

FIG. 3A illustrates a burst comprised of two laser light pulses, withdifferent polarization vectors, that are separated in time.

FIG. 3B illustrates the polarization vectors of four pulses that areseparated in time.

FIG. 4A illustrates a burst comprised of two laser light pulses, withdifferent polarization vectors, that are overlapped in time.

FIG. 4B illustrates the polarization vectors of three pulses that areoverlapped in time.

FIG. 5 is a schematic drawing of a non-limiting embodiment of a laserprocessing system.

FIG. 6 is a schematic of a laser means that outputs bursts of pulsesthat embody various characteristics according to the present invention.

FIG. 7 is a schematic drawing of a non-limiting embodiment of a laserdevice for the laser processing system.

FIG. 8 is a schematic drawing of a non-limiting embodiment of an opticalfrequency conversion device for the laser processing system.

FIG. 9 is a schematic drawing of a non-limiting embodiment of an opticalfrequency conversion device for the laser processing system.

FIG. 10 is a schematic drawing of a non-limiting embodiment of a beammanipulation device for the laser processing system.

FIGS. 11A and 11B are schematic drawings of non-limiting embodiments ofcylindrical lens telescopes for the laser processing system.

FIG. 12 is a schematic drawing of a non-limiting embodiment of targetsubstrate mounting device for the laser processing system.

FIG. 13 is a schematic drawing of a non-limiting embodiment of a laserprocessing system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In order to realize the ultra-high precision possible with femtosecondlaser materials processing, it is necessary to monitor and regulatelaser performance and process parameters in real time and with highprecision. The present invention incorporates dynamic diagnostics withcontrol devices, allowing for active “real time” manipulation of theincident ultrashort laser pulses synchronized with “real time”manipulation of the target substrate. By coordinating sensor input andcomponent control, it is possible to adjust laser performance in orderto counteract relatively long term variations associated with changingambient conditions (temperature, humidity, etc.) and/or laserburn-in/degradation. Furthermore, the laser processing system is capableof rapid adjustment so that the system performance can be programmed toperform a predefined set of interrelated tasks and monitor those tasksas they are being performed within predefined design tolerances. Systemcontrol would allow for real time manipulation of the laser performanceand beam characteristics relative to target motion. For example, thelaser device would be capable of varying laser repetition rate andoutput power (which can be tied to target translation velocity in orderto keep pulse overlap and/or the rate of laser dosage constant duringthe fabrication of 3D patterns). The laser system further allows foractive variation and control of the optical frequency (frequencies)incident on the target via an optical conversion device. In addition,the system provides for variation of size and shape of the focal regionand the direction of the incident laser polarization. Finally, thevariation of system parameters (laser device output, frequencyconversion and beam manipulation) can be tied directly to targetmanipulation, thereby significantly enhancing the user's ability toprecisely vary and control the laser/material interaction conditions.

A detailed description of the preferred embodiments of the inventionwill now be given referring to the accompanying drawings.

Referring to FIG. 1A, the temporal arrangement of multi-pulses isdepicted, where only two separated pulses that comprise a burst areillustrated for purposes of clarity. A burst might comprise more thantwo pulses, but for reasons of drawing clarity, only two pulses areshown. The first pulse 11 has the parameters of pulse energy pe₁ andpulse width pw₁ and the second pulse 12 has the parameters of pulseenergy pe₂ and pulse width pw₂. The pulses are separated by timeseparation t_(s). Preferably, the time separation t_(s) has a value thatis much greater than the pulse widths pw₁ and pw₂, i.e., pw₁ orpw₂<<|t_(s)|. Depending upon the particular application, the pulse widthvalues for pw₁ and pw₂ may or may not be equal, and the pulse energyvalues for pe₁ and pe₂ may or may not be equal. The pulse width valuespw₁ are generally in the nanosecond range, and the pulse width valuesfor pw₂ are generally in the picosecond to femtosecond range.

The temporal pulse shape shown in FIG. 1A has a Gaussian shape, but itis not limited to Gaussian shapes. The pulse shape is defined moreappropriately here by pulse width and peak power. The relationship ofthe multiple pulses is characterized by pulse width, peak power andseparation time between pulses. Separation time is measured as apositive value from the center of the long pulse as a delay time afterthe long pulse.

The pulse shape and the location of two pulses as shown in FIG. 1A canbe used as envelopes for the peaks of more than two pulses. Multiplepulses enclosed in the envelope defined by the first pulse shown in FIG.1A will cause a similar effect. For example, the long pulse's shapecurve serves as an envelope of peak power of each of the pulses enclosedwithin it.

Preferably, the separation time of this coupling of two pulses isbetween −pw₁ and +pw₁, where pw₁ is a pulse width of the long pulse. Forexample, when pw₁=3.0 nanoseconds, it is between −3.0 nanoseconds and+3.0 nanoseconds. The peak power of long pulse is less than that of theshort pulse to avoid damage to the surrounding area by the long pulse.Energy of each pulse is between 0.0001 microjoules and 10 microjoules.

Referring to FIG. 1B, two overlapping pulses that comprise a burst areillustrated. A burst might comprise more than two pulses, but forreasons of drawing clarity, only two pulses are shown. The pulses 13, 14shown in FIG. 1B are identical to pulses 11, 12 illustrated in FIG. 2A.Depending upon the particular application, the pulse width values forpw₁ and pw₂ may or may not be equal, and the pulse energy values for pe₁and pe₂ may or may not be equal. The pulse width values pw₁ aregenerally in the nanosecond range, and the pulse width values for pw₂are generally in the picosecond to femtosecond range. In this embodimentof the invention, pulses 13 and 14 overlap in time. As discussed in moredetail below, pulses can be overlapped to achieve specific materialsdamage and/or ablation.

In another aspect of the invention, the wavelength of successive pulsesin a burst can be changed. Referring to FIG. 2A, two separated pulsesthat comprise a burst are illustrated. A burst might comprise more thantwo pulses, but for reasons of drawing clarity, only two pulses areshown. The first pulse 21 has the parameters of pulse wavelength wl₁ andpulse width pw₁ and the second pulse 22 has the parameters of pulsewavelength wl₂ and pulse width pw₂. The pulses are separated by timeseparation t_(s). Preferably, the time separation t_(s) has a value thatis much greater than the pulse widths pw₁ and pw₂, i.e., pw₁ orpw₂<<|t_(s)|. Depending upon the particular application, the pulse widthvalues for pw₁ and pw₂ may or may not be equal, and the pulse wavelengthvalues for wl₁ and wl₂ are never equal. The pulse width values pw₁ aregenerally in the nanosecond range, and the pulse width values for pw₂are generally in the picosecond to femtosecond range.

Referring to FIG. 2B, two overlapping pulses that comprise a burst areillustrated. A burst might comprise more than two pulses, but forreasons of drawing clarity, only two pulses are shown. The pulses 23, 24shown in FIG. 2B are identical to pulses 21, 22 illustrated in FIG. 2A.Depending upon the particular application, the pulse width values forpw₁ and pw₂ may or may not be equal, and the wavelength values for wl₁and wl₂ are never equal. In this embodiment of the invention, pulses 23and 24 overlap in time. The pulses can be overlapped to achieve specificmaterials damage and/or ablation.

It is well known that the materials respond differently to the radiationof different wavelengths, and absorption of laser energy dependsstrongly on wavelength. A change in a material's property changes theway a material responds to laser light of a particular wavelength. Thewavelength of each successive pulse in a burst is tailored to interacteffectively with the material in response to the changes caused by thewavelength of the prior pulse. The wavelength of second pulse istailored such that better coupling of the laser beam of the second pulseand the material modified by the first pulse is achieved.

In another aspect of the invention, the polarization of the pulsescomprising a burst is changed. Polarization of the laser pulse affectsthe surface structure of the modified material. For example, a linearlypolarized laser beam creates a wavy pattern on the machined surface andan elliptically drilled hole. The orientation of wavy pattern depends onthe direction of polarization. In some applications, the cut must bevery smooth or the portion remaining after vaporization must be a smoothand flat surface (e.g., chip repair and micro-fluidic devices). Rapidrotation of the polarization direction of the laser pulse homogenizesresults in a smooth surface. In another application, a wavy pattern canbe made on wear resistant parts such as friction parts. In thoseinstances, the wavy pattern can be tailored by changing the polarizationdirection.

Referring to FIG. 3A, two pulses, separated in time, which comprise aburst are illustrated. A burst might comprise more than two pulses, butfor reasons of drawing clarity, only two pulses are shown. The firstpulse 31 has the parameters of pulse polarization plz₁ and pulse widthpw₁ and the second pulse 32 has the parameters of pulse polarizationplz₂ and pulse width pw₂. The pulses are separated by a time separationt_(s) (not shown). Preferably, the time separation t_(s) has a valuethat is much greater than the pulse widths pw₁ and pw₂, i.e., pw₁ orpw₂<<|t_(s)|. Depending upon the particular application, the pulse widthvalues for pw₁ and pw₂ may or may not be equal, and the pulsepolarization values for plz₁ and plz₂ are never equal. The pulse widthvalues pw₁ are generally in the nanosecond range, and the pulse widthvalues for pw₂ are generally in the picosecond to femtosecond range.

Referring to FIG. 3B, a different view of the polarization of the pulsescomprising a burst is shown. The burst in FIG. 3B comprises fourseparate pulses, and each pulse has its own polarization value, i.e.,plz₁, plz₂, plz₃ and plz₄. The four polarization factors are arrangedapart from one another as shown in FIG. 3B.

Referring to FIG. 4A, two overlapping pulses that comprise a burst areillustrated. A burst might comprise more than two pulses, but forreasons of drawing clarity, only two pulses are shown. The pulses 41, 42shown in FIG. 4A are identical to pulses 31, 32 illustrated in FIG. 3A.Depending upon the particular application, the pulse width values forpw₁ and pw₂ may or may not be equal, and the pulse polarization valuesfor plz₁ and plZ₂ are never equal. In this embodiment of the invention,pulses 41 and 42 overlap in time. As discussed in more detail below,pulses can be overlapped to achieve specific materials damage and/orablation.

Referring to FIG. 4B, a different view of the polarization of the pulsescomprising a burst is shown. The burst in FIG. 4B comprises fourseparate pulses, and each pulse has its own polarization value, i.e.,plz₁, plz₂ and plz₃. The three polarization factors are arranged closeto one another as shown in FIG. 4B.

It is well known that for a given direction of laser beam polarization,a particular texture is generated on an impinged-upon material. Thepolarization is changed from pulse to pulse in the same manner asdescribed above for the wavelength in the course of electronic andphysical change of material caused by the successive interaction oflaser pulse with material to achieve the best result. Rapid change ofpolarization also homogenizes texture of the impinged-upon region.Direction of polarization is manipulated with respect to the crystalorientation to achieve maximum laser-matter interaction.

Referring to FIG. 5, an embodiment of the present invention is shown.The laser means 51 is controlled by a control means 53. The output ofthe laser means 51 is directed to a beam manipulation means 52, whichthen outputs an output beam. The output beam may comprise a series ofpulses having the characteristics as described in FIGS. 1A to 4B andtheir accompanying text. For example, the output beam might comprise aseries of composite pulses (i.e., a pulse comprised of two of morepulses overlapped in time or spaced very closely in time, such as inFIGS. 1A and 1B) having a varying repetition rate, wherein the timebetween the composite pulses is varied. The function of the laser means51 is to provide laser pulses with ultrashort pulse duration forapplication to a target substrate. Preferably, the laser means 51 is anamplified fiber laser system with ultrashort pulse duration, from 10femtoseconds to 1 picosecond. The fiber laser preferably has afundamental output wavelength in the near IR range, from 1-2 microns.The fiber laser has a variable repetition rate from 10 kilohertz to 50megahertz, and with an output pulse energy range from 100 nanojoules to100 millijoules.

As discussed above, a burst of multiple pulses with differentwavelengths, different pulse widths and different temporal delays may bedesired. Referring to FIG. 6, an embodiment of the laser means 51 isillustrated, which increasing the increasing the possible energy andaverage power from ultrafast fiber lasers. A longer pulse envelope canbe obtained by utilizing a series of chirped gratings that reflect atdifferent wavelengths. After amplification, a similar series of gratingscan be placed to recombine/compress the pulses. In FIG. 6, pulses from afemtosecond pulse source are passed through an acousto-optic modulator,a polarized beam-splitter and a Faraday rotator, and are then suppliedto a series of chirped fiber stretcher gratings that operate ondifferent portions of the input pulse spectrum. The spacings between thestretcher gratings can be 1 ₁, 1 ₂, 1 ₃ . . . . In order to reconstructthe pulses after amplification (in, e.g., a Yb amplifier), the spacingsbetween a series of complementary bulk glass Bragg grating compressorsare set to n1 ₁, n1 ₂, n1 ₃, . . . , where n is the refractive index ofthe fiber between the stretcher fiber gratings, assuming that the bulkBragg compression gratings are separated by air. The reconstructed pulseis output via a second beam splitter. As previously mentioned, thereconstructed pulse is generally the result of incoherent addition ofthe separately amplified spectral components of the input pulse.

If the distances between the compression and stretcher gratings are notequalized as described above, then multiple pulses will appear at theoutput. If the distances are not equal between the different sectionsthan the temporal delays will not be equal. This can be beneficial forapplications such as micro-machining. By varying the stretching andcompression ratios, pulses with different pulse widths can be generated.A single broadband compression grating can be used when generatingmultiple pulses.

A regenerative amplifier is an alternative source for generatingultrashort pulses for micro-machining. The methods of pulse shapingdescribed here can work in the regenerative amplifier in some cases.However, the regenerative amplifier is not as flexible as the fiberamplifier system for modification of the pulse shape. For example, longpulse widths are limited to repetitive features equal to the round triptime of the regenerative amplifier, e.g., approximately 10 nanoseconds.For a regenerative amplifier, the pulse train created by the gratingsneeds to be less than the round trip time of the regenerative amplifier.

As shown in the embodiment illustrated in FIG. 5, the control means 53is coupled to the laser means 51. The control means 53 monitors severaloutput laser parameters, such as the average output power, the pulsetrain (repetition rate and/or burst mode structure), pulse duration(and/or temporal phase, i.e., FROG), and spatial phase (wavefrontsensor). The monitored parameters are linked to the control means 53 inorder to vary laser performance (pulse energy, repetition rate and pulseduration) through feedback loops. Furthermore, the feedback loops couldbe linked to compressor alignment (e.g., grating separation) in order topre-chirp the laser pulse, thereby compensating for the opticaldispersion caused by the components in subsequent laser system modules.The control means 53 may comprise, for example, a desktop computer, alaptop computer, a tablet computer, a handheld computer, a workstationcomputer or any other computing or communicating device. The controlmeans 53 may execute any of the well-known MAC-OS, WINDOWS™, UNIX, LINUXor other appropriate operating systems on a computer (not shown). Thecontrol means 53 might be networked to other computing means by physicallinks or wireless links. The control means 53 may comprise an inputdevice, an output device, random access memory (RAM) and read-onlymemory (ROM), a CD-ROM, a hard drive, or other magnetic or opticalstorage media, or other appropriate storage and retrieval devices. Thecontrol means 53 may also comprise a processor having a system clock orother suitable timing device or software. The input device mightcomprise a keyboard, mouse, a touch screen, pressure-sensitive pad orother suitable input device, and the output device can comprise a videodisplay, a printer, a disk drive or other suitable output device.

In the embodiment shown in FIG. 7, several elements comprise the beammanipulation means 52. These elements are exemplary, and are notintended to be limiting of the types of elements that can comprise thebeam manipulation means 52. In the embodiment shown in FIG. 7, a powermeter 72 monitors the average power of the raw beam 75 output from thelaser means 51. The average power measurement data is sent to thecontrol means 53. Photodiode 73 monitors the pulse repetition rate ofthe raw beam 75 output from the laser means 51, and the photodiode 73coupled to the control means 53. A FROG device 74 measures the pulseduration of the raw beam 75 and the pulse duration measurement data issent to the control means 53. The elements shown for the beammanipulation means 52 (powermeter 72, photodiode 73, FROG 74) alsoprovide go/no-go measurements. If the laser means output does not meetpreprogrammed specifications, a command is given to cease processing(e.g., close a shutter and stop target motion control). For activecontrol, the photodiode 73 would be linked to the downcounter (forchange of the repetition rate within the final amplifier), thepowermeter 72 would be linked to the pump diode current and/or anexternal attenuator; and the FROG could be linked to control thecompressor gratings (i.e., the adjustments of the compressor alignmentwould generally be pre-programmed). For example, if a particular pulseenergy was required at a variety of repetition rates (e.g., toaccomplish a curved cut with constant pulse overlap), the photodiode 73would be linked to the downcounter, and it would also be linked to thecompressor gratings, since the separation of the compressor gratings hasto be changed to maintain minimum pulse duration with changingrepetition rate. Likewise, the pump current might have to be increased(as the repetition rate is increased), or the beam attenuation wouldhave to increase (as repetition rate is decreased) in order to keep theaverage power divided by the repetition rate at a constant value.

An external modulator (acousto-optic and/or electro-optic) could be usedto achieve finer control of the amplitude of pulses within a burst ofpulses. Since the modulation efficiency is known before hand, it shouldbe possible to program the driving voltage on the modulator in order toproduce a particular burst structure. The burst structure would have tobe monitored with a separate calibrated photodetector in order to checkthat the desired pulse structure was being produced. If there wassufficient mismatch between the program and the measured output, then ashutter would have to block the beam and processing would have to beterminated.

Referring to FIG. 8, another element that may comprise the beammanipulation means 52 is illustrated. This element of the beammanipulation means 52 performs optical frequency conversions, and is anoptional element, as are the previously described elements. This elementof the beam manipulation means 52 may comprise a nonlinear opticalcrystal 81 (or crystals) and optics 82 that function to convert aportion of the fundamental laser frequency to higher order harmonics(particularly 2^(nd) and 3^(rd) order harmonics). The nonlinear opticalcrystal unit 81 outputs crystal temperature and angle data to thecontrol means 53. The optics 82 receive control inputs (angle α, angleβ) from the control means 53 that determines the angle of the opticswith respect to the incident beam. The means for converting an opticalfrequency can further comprise a spectrometer 83.

Referring to FIG. 9, another element that may comprise the beammanipulation means 52 is illustrated. The raw beam is received bytelescope 96, which comprises two lenses 96A, 96B that function to focusthe beam within the non-linear crystal (NLO) 97 and to collimate theoutput of the telescope 96. The NLO 97 further comprises a controlsystem (not shown) for providing functionality such as crystal rotation,crystal translation and temperature control. The control systeminterfaces with the control means 53. FIG. 9 further illustrates optics98A, 98B for separating the harmonic beam from the fundamental beam.Although the embodiment illustrated in FIG. 9 shows that the beams areseparated, the harmonic beam(s) do not need to be separated from thefundamental beam at this stage. If the harmonic beam(s) are separatedfrom the fundamental beam, they may be combined for subsequent use.

Through manipulation of the NLO 97 (position, rotation, and temperature)and the performance of the laser means 51 (pulse energy, repetitionrate, and pulse duration), the input fundamental laser power and theharmonic conversion efficiency can be optimized. This would beparticularly useful in applications in which two or more harmonics ofthe laser are coordinated for processing. For example, if one needed torapidly vary the ratio of fundamental and 3^(rd) harmonic on a targetsubstrate, one could modulate the pulse energy and/or pulse duration.This changes the laser intensity incident on the NLO 97 and thereforequickly changes the harmonic conversion efficiency. Furthermore, if TypeI phase matching were used, polarization could be used to separate(e.g., using a Glan/Thompson polarizing beam splitter) and/or modulate(e.g., using an electro-optic Pockels cell before the crystal to rotatethe polarization of the fundamental) the different harmonics. Changes inthe conversion efficiency could be accomplished by changing the inputlaser polarization and/or by changing the incident pulse duration byvarying the grating spacing within the compressor.

Referring to FIG. 8, the spectrometer 83 has a calibrated amplitudescale to be used to monitor the power of the fundamental and harmonicbeams, harmonic conversion efficiency and extinction ratio if thewavelengths need to be separated for a particular application. Thesignal from the spectrometer 83 could then be fed back into the controlsystem of the NLO 87 to adjust the angle or temperature of the NLO 87.Alternatively, the signal from the spectrometer 83 could be fed to thecontrol means 53 to control changes to the laser output (pulse energy,repetition rate and/or pulse duration). In general, the desiredconversion efficiency would not determine the laser output parameters.Instead, the conversion efficiency would have to adjusted to compensatefor the changes in pulse energy as described above. For example, if aconstant conversion efficiency were desired while changing pulse energy,the angle or temperature of the NLO 87 would have to be adjusted. Thedesired conversion efficiency would have to be predetermined andcoordinated with the measured pulse energy and pulse duration. Theconversion efficiency would then be confirmed by measurement with thediagnostic spectrometer.

Referring to FIG. 10, another element of the beam manipulation means 52is illustrated. As discussed previously, this element of the beammanipulation means 52 is optional, and can be used with the otheroptional elements. This element of the beam manipulation means 52manipulates the size, shape, divergence and polarization of the laserpulses, as well as to direct the position of the laser pulses on thetarget substrate. Preferably, this element of the beam manipulationmeans 52 comprises two cylindrical lens telescopes 101, 102 and steeringoptics 103. The steering optics 103 direct the shaped pulses to afocusing means, which focuses the shaped pulses. The steering optics(not shown in detail) may comprise, but are not limited to, apiezoelectric scanning mirror, a galvanometric scanning mirror, anacousto-optic deflector or an electro-optic deflector.

For illustration purposes a free space optical system is shown, howeverfiber delivery could also be employed in part or completely. The fiberdelivery system would improve beam pointing stability, however the fiberdamage threshold would limit the peak output power. This element of thebeam manipulation means 52 may further comprise a beam profiler 104 tomonitor and measure beam shape, size and divergence. A CCD camera workswell as a beam profiler, and the measurement results are sent to thecontrol means 53. By moving the beam profiler 104 axially along the pathof the beam as it travels through focus, the near field beam profile(calibrated to an appropriate image plane, such as the entrance to thefocal objective) can be determined, and the beam divergence (bymeasuring M²) along x and y axes can be measured. The beam profiler 104could be integrated with motorized translation stages controlling theoptical alignment of the elements within the beam manipulation modulevia feedback loops to dynamically change the beam characteristicssimultaneously (beam size, shape, divergence, polarization) and/or tomonitor and confirm that the system is performing according topreset/programmed performance parameters. In addition, polarization canbe measured via the ratio of reflected vs. transmitted light through aGlan/Thompson polarizing beam splitter (not shown). A beam positiondetector 105 is used for the detection of beam position (if beamscanning is employed), with the measurement results being sent to thecontrol means 53. A CCD camera works well as a beam position detector.

This element of the beam manipulation means 52 would generally notinfluence the laser output parameters. However, if a particular fluenceneeded to be maintained while the irradiated area (as determined by thebeam size and shape) was changed, the pulse energy would have to becoordinated with the beam size. As such, the CCD beam profiler 104 wouldbe linked to a software system, which would calculate the beam area onfocus. A change in the beam size would then be linked to the pulseenergy, so that a decrease in beam area would be coordinated with acorresponding increase in the beam attenuation (i.e. pulse energy) orvise versa (although it would be more difficult to increase the pulseenergy).

The two cylindrical beam-expanding telescopes 111, 112 provide maximumflexibility in the manipulation of beam size, shape and divergence.Referring to FIG. 11A, a three-element cylindrical beam-expandingtelescope (CLT1) provides variable beam magnification (or reduction)from 2× to 8× and variable divergence angle. The adjustment of the lensseparation between lens 111A and lens 111C provides for control oftelescope magnification. The adjustment of the position of lens 111Bprovides for control of beam divergence. The control means 53 controlsthe separation of lens 111A and lens 111C, as well as the positioning oflens 111B therebetween. FIG. 11B shows a three-element cylindricalbeam-expanding telescope (CLT2) that is identical to the three-elementcylindrical beam-expanding telescope shown in FIG. 11A, except it isoriented ninety degrees with respect to CLT1. With two such cylindricallens telescopes oriented at 90° to one another, it is possible toindependently control beam size and divergence along orthogonal beamcross-sectional axes, thereby allowing for uniform expansion of a roundbeam (e.g., 2× to 8× magnification) or astigmatic expansion (with amaximum ellipticity of 4:1, assuming collimated output). As such, thisbeam manipulation means allows for correction of undesiredellipticity/astigmatism in the raw beam as well as the ability toproduce a specifically tailored beam shape for direction to the target.Greater amounts of beam ellipticity could be achieved if the twocylindrical lens telescopes 101, 102 can be controllably rotated aboutthe z-axis, such that the angle between the optical axis of CLT1relative to that of CLT2 can be varied from ninety degrees to zerodegrees. For example, if the angle between CLT1 and CLT2 were zerodegrees, a narrow line with controllable width (dependant upon variablemagnification settings). Such a beam could be used to produce a linefocus for a variety of applications described in detail in latersections.

The steering optics 103 direct the shaped beam to a focusing means (seeFIG. 13), which focuses the shaped beam. The steering optics (not shownin detail) may comprise, but are not limited to, a piezoelectricscanning mirror, a galvanometric scanning mirror, an acousto-opticdeflector or an electro-optic deflector.

The means for focusing comprises a focusing optic, having (but notlimited to) a high NA objective, producing sub-10 micron focal diameterson the target substrate. The focusing optic comprises refractive and/orreflective optics that are known to one of skill in the art. The laserprocessing system requires that the focusing optic have the propertiesof maximum optical transmission, maximum NA and minimal opticalaberration. In addition, the means for focusing has to be matched to thelaser means 51.

For illustration purposes a free space optical system is shown, howeverfiber delivery could also be employed in part or completely. The fiberdelivery system would improve beam pointing stability, however the fiberdamage threshold would limit the peak output power.

Referring to FIG. 12, an exemplary positioning means is illustrated. Thefunction of the positioning means is to precisely position the targetsubstrate for impingement by the focused beam. In an embodiment of thelaser procession system, the positioning means comprises a registeredmounting surface 122 providing a reference for target substrate movementrelative to the focused beam. When used with scanning mirrors, thepositioning means provides reference with movement of the incident beamrelative to the target. The positioning means also provides coordinatedmovement of both the target substrate and the incident beam relative tosome reference point defined by the rest position of the registeredmounting surface 122. In one embodiment, the motion controller 123 wouldprovide 6-axis motion control capable of simultaneous movement with highpositioning accuracy and resolution. The registered mounting surface 122would also include a vacuum system (not shown) connected to theregistered mounting surface 122, where the pressure is applied through ascreen so that sufficient force is available to hold the targetsubstrate. The screen provides sufficient support for the targetsubstrate so that it does not deform under the pressure. Furthermore, aheating/cooling element (not shown) could be integrated into theregistered mounting surface 122 to allow for control of the targetsubstrate temperature, as well as providing for the monitoring of thetemperature of the target substrate. The registered mounting surface 122and the motion controller 123 operate under the control of the controlmeans 53.

In an alternative embodiment, a chamber 121 encloses the targetsubstrate, the registered mounting surface 122 and the motion controller123, allowing for control of atmospheric pressure and gas content. Thechamber 121 and attached air systems would allow for some range ofpressures, both vacuum and over pressure. As shown in FIG. 12, thechamber 121 can allow for the control of the atmospheric contents withthe incorporation of lines for the introduction of appropriate fillgases. In addition, sensors may measure the pressure and gas compositioninside the processing chamber (not shown).

Additional tools may be included to monitor the status of the targetsubstrate, and to confirm/control the focal position relative to thesurface of the target substrate. For example, an illumination andoptical microscopic viewing system (not shown) could be used to locatealignment markers, confirm/deny laser damage, and measure laser affectedfeature volume and morphology. Additional data could be obtained byincluding spectroscopic diagnostics such as laser induced breakdownspectroscopy (LIBS) or laser-induced fluorescence. A range-finding toolthat precisely determines the distance from the target surface to thefocal point could also be employed. This distance is critical since thepreferred application of the present invention is micron-level materialprocessing. Use of a camera system that images the surface of the targetsubstrate could be used as well. At these dimensions, extremely smallerror/uncertainty will corrupt the user's ability to precisely controlthe laser/material interaction. This can be particularly complicatedsince several such applications potentially involve sub-surfaceprocessing of materials with non-planar surfaces. Signals from theviewing/spectroscopic tools could feedback to other system components(e.g., the control means, the means for converting optical frequencies,etc.) to precisely influence the extent and nature of the laser/materialinteraction. Furthermore, the signal from the range finding tool and theviewing/spectroscopic tools can be fed back to the registered mountingsurface 122, the steering optics 93 and the control means 53 to ensurethat the beam is accurately delivered to the target substrate.

Referring to FIG. 13, an embodiment of the present invention isillustrated, which comprises control means 53, a laser means 51, anoptical frequency conversion means 86, telescopes 101, 102 steeringoptics 93, a focusing means 136 and positioning means 123. Under thecontrol of the control means 53, a raw beam is emitted from the lasermeans 51 and is received by the optical frequency conversion means 86,which outputs a frequency-converted beam. The frequency-converted beamcan comprise, for example, the fundamental laser frequency, a harmonicof the fundamental laser frequency, a combination of both fundamentaland harmonic laser frequencies, or a non-harmonic frequency. Thefrequency-converted beam is then processed by the telescopes 101, 102and steering optics 93, which comprise optics that change the beam'ssize, shape, divergence and polarization as well as other optics thatdirect the beam to the target. The telescopes/steering optics output ashaped beam to the focusing means 136, which focuses the shaped beaminto a focused beam for impingement upon the target substrate. Thepositioning means 123 holds the target substrate at specific anglesand/or positions, and can also maintain the target substrate at aspecific temperature. In addition, the positioning means 123 canmaintain the target substrate in a specific environment (i.e., pressure,gas, etc.).

The control means 53 is coupled to the laser means 51, the opticalfrequency conversion means 86, the telescopes 101, 102, the steeringoptics 93, the focusing means 136 and the positioning means 123 via aplurality of data/control lines 131, 132, 133, 134, 135. Each of thedata/control lines transmits control information from the control means53 to its respective means of the laser processing system. Each of thedata/control lines can be physical links to the laser means 51, theoptical frequency conversion means 86, the telescopes 101, 102, thesteering optics 93, the focusing means 136 and the positioning means123, or each of the data/control lines can be wireless links to thevarious means, or a mixture of both physical links and wireless links.In addition, the control means 53 receives status and diagnosticinformation from the respective means of the laser processing system viathe data/control lines.

One application for the laser processing system is refractive indexmodification, i.e., waveguide writing and phase mask, holograph anddiffractive optic element fabrication. Several publications havedemonstrated that irradiation by ultrashort laser pulses (<1 picosecondpulse duration, typically<10 millijoules/pulse energy) can change theoptical properties of a transparent (to the incident laser radiation)substrate without forming damage sites. Most publications thus far havefocused upon inducing a refractive index increase in order to directlywrite embedded waveguides (typically in, but not limited to, glasssubstrates). In this case, light can be guided within exposed regionsvia total internal reflection in much the same way as in standardoptical fiber. For most such applications, a transverse writing geometryis preferred because it allows for the greatest flexibility in thefabrication of complicated three-dimensional structures. Typically,laser scanning is impractical, so the target substrate is moved relativeto a fixed laser focal position. It has been noted that exceeding awell-defined laser intensity threshold results in ablation of the glasssubstrate, rather than modification of the refractive index.Furthermore, in order to induce the maximum change in refractive index,it is typically necessary to expose the target to many laser pulseswhile keeping the laser intensity just below the ablation threshold.Therefore, in order to insure controllable device fabrication it isnecessary to precisely control the laser intensity at focus. Thisrequires active verification of laser performance (pulse energy, pulseduration, beam size, beam shape, and beam divergence), activeregistration of the position of beam focus (in three dimensions)relative to the target substrate surface, and monitoring processingstatus with an independent viewing system. As the motion of the targetsubstrate becomes more complex, control of the laser output (pulseenergy, laser repetition rate, and pulse duration) must be matched tosample motion in order to control single pulse fluence and total laser“dose” (single pulse fluence multiplied by an overlap factor). As such,this application requires the coordination among all the opticalcomponents in the laser system as described in this patent. Furthermore,control of the beam characteristics (beam size, repetition rate, shape,divergence and polarization) can also be linked to target motion. Thisallows the cross-section (size and shape) of the transversely writtenwaveguide to be varied relative to its axial length, making it possibleto fabricate waveguides of optimal dimension (10 μm diameter roundwaveguides are standard for single-mode telecommunication applications)as well as to incorporate periodic structures (such as distributed Braggand long period gratings) and/or tapered sections (for couplers, modefilters, etc.) into three-dimensional waveguide devices. To date, suchfeatures have been produced using multiple writing steps, however thedescribed laser system allows for greatly increased processingefficiency and reduced fabrication time by reducing the fabrication ofsuch devices to a single writing step. Furthermore, the polarization ofthe writing beam has been shown to produce birefringent structures viaanisotropic refractive index modification allowing for the incorporationof polarization maintaining/discriminating elements into such devices.Precise position for writing elements such as periodic structures andcouplers is necessary. This is only possible when the laser position isprecisely known. Excessive beam wander cannot be tolerated. The use of afiber laser system virtually eliminates fluctuations in beam pointingassociated with changes in temperature and can provide pointingstability of <±10 micrometers whereas the current standard for aTi:Sapphire regenerative amplifier is <±25 micrometers (Positive LightIndigo).

Although the majority of relevant research has been focused uponwaveguide writing in glasses, “non-destructive” refractive index (orother optical property) modification has other potential applications,which have yet to be realized due to the general lack of control overprocessing parameters. Such applications include fabrication ofdiffractive optic elements and phase masks by precisely changing therefractive index of localized “pixels.” Two-dimensional andthree-dimensional arrays of such point modifications can be used toproduce transmission optics such as phase correction plates, Fresnellenses, gratings, etc. Furthermore, more complicated holographicstructures can be fabricated with this technique. As with waveguidewriting, these applications require the precise monitoring andcoordination of laser performance (pulse energy, laser repetition rate,and pulse duration), beam characteristics (beam size, shape, divergence,and polarization), and target motion.

Another application for the laser processing system is surfacemicro-marking and sub-surface micro-marking. At intensities sufficientto cause damage (material ablation and/or cracking), pulsed lasers arecommonly used to produce visible marks on and/or below the materialsurface for labeling. Femtosecond lasers allow for greater control inthe production of very small features (<10 micrometers) with minimumcollateral damage. Such precision is particularly necessary whenavailable space is limited, labeling stealth is desired and/or thetarget substrate is fragile (due to material brittleness or small size).The present invention's laser monitors/controls (wavelength (viaharmonic conversion), pulse energy, repetition rate, and pulseduration), and beam monitors/controls (beam size, shape, divergence, andpolarization) relative to target position can be used to dynamicallycoordinate the rate/nature of energy deposition, the rate/nature ofmaterial modification and focal volume/shape relative to the position ofthe focal spot in the target substrate (via target translation and/orbeam scanning).

Another application is the texturing of surfaces through to productionof raised and/or recessed areas. Precise relief structures, with afeature height of less than ten micrometers, have been shown to modifythe coefficient of friction in some materials. Such structures have beenused to relieve stiction and even aid in lubrication, thereby allowingfor smoother motion and decreased wear for moving parts. Thisapplication is particularly attractive for relatively small parts(micro-motors, hard drives, mini-disk players) where the application oflubricants may be impractical and thermal management (reduction of heatfrom friction) is critical. Although surface micro-structuring ispossible with conventional nanosecond lasers, much greater repeatabilityand depth precision is possible using an ultrashort pulse laser systemdue to the mechanics of Coulomb explosion. The present invention's lasermonitors/controls (wavelength (via harmonic conversion), pulse energy,repetition rate, and pulse duration), and beam monitors/controls (beamsize, shape, divergence, and polarization) can be used in much the sameway as with micro-marking to produce arrays of precisely shaped reliefstructures. Similarly, surface contouring could be achieved by texturingcontinuous surface areas.

A related application is the fabrication of precise trenches and groovesin a variety of materials. For example, extremely precise trenches in Siare required for a variety of microelectronic applications. Severalresearch groups have demonstrated that the best results are obtainedusing femtosecond laser pulses with the laser intensity just above theablation threshold. The laser controls and diagnostics incorporated inthis laser system allow for active control of processing parameters inorder to insure that the laser intensity remains within the optimalrange thereby assuring consistent feature size, material removal rateand thermal effect. In addition, the ability to control the size, shape,divergence and polarization of the beam makes it possible to furtheroptimize the shape and edge quality of such grooves and trenches. Forexample, it has been demonstrated that the use of a highly ellipticalbeam with its major axis parallel to the direction of translation iscapable of producing trenches with higher aspect ratio and bettersurface quality than is possible using a round focal beam Adjusting thelaser polarization relative to the direction of scanning has also beenshown to affect the surface and edge quality of femtosecond machinedgrooves. The ability to actively monitor and independently control laserand beam parameters, as enabled by this laser system, is essential toreproducible micron-level precision in the fabrication of surfacegrooves and trenches.

Another application for the laser processing system is hole, channeland/or via fabrication (for, e.g., electronics, photonics ormicro-fluidics). Channel or continuous voids, with cross-sections from1-100 microns and millimeter to centimeter length, are interesting in avariety of photonic and micro-fluidic devices. For example, “holey”fibers are emerging in several novel photonic applications such asdispersion management and continuum light generation. Channelfabrication is also critical to chemical, biological and medical devicesutilizing micro-fluidics (both liquid and gas).

Photonic bandgap and holey fibers are currently produced usingfiber-pulling techniques. However, these are limited in their ability tosupport structures with variation along their length. U.S. Pat. No.6,654,522 illustrates a holey fiber device where the entire length isless than twenty centimeters and where a few centimeters may besufficiently long. It is not efficient to make such a device by a fiberpulling method optimized for making fibers that are kilometers in lengthat a time, especially during prototyping and device optimization. Such adevice may be more appropriately fabricated utilizing the ultrafastlaser processing system of the invention. A mode filter or a mode-tapercould be fabricated from a few millimeter long solid piece of glass thatis fused on the end of the fiber. An additional application is thefabrication of photonic crystal waveguides, as described M. Augustin,“Highly efficient waveguide bends in photonic crystal with a lowin-plane index contrast,” Optics Express, Vol. 11, pp. 3284-9, (2003).An additional application is the fabrication of high pulse energy fiberamplifiers. Fibers in optical amplifiers are typically tens of meters inlength. However, when multimode optical amplifiers have been used foramplifying single mode beams, the area is increased. This allows formore active ions per unit length. Thus, such amplifiers are typicallymore than ten times shorter (about one meter). Multimode holey fiberamplifiers are possible that could support even larger beams. With theseamplifiers, the length could be in the ten centimeter range and it maybe more desirable to make these structures by ultrafast microprocessing.This could also include a glass extrusion process.

Currently, the preferred method for micro-fluidic device fabrication isvia lithographic processing, often involving several cycles of UV lightexposures that is followed by a solvent etch. Femtosecond lasers arecapable of directly machining blind and through holes of modest aspectratio (1:10-1:100, depending upon substrate material, laser parametersand hole diameter).

Another technique for creating a fluidic channel is based upon the factthat femtosecond laser irradiation increases the susceptibility ofseveral glasses to etching by HF acid. Fabrication of complicatednetworks of channels and holes has been demonstrated using a directwrite exposure technique followed by HF acid etch (in much the same waythat lithographic techniques are currently used with photosensitiveglasses such as Foturan®). It has also been demonstrated thatetching/material removal can be accomplished during irradiation withfemtosecond laser pulses. In this case, a mildly acidic solutionadjacent to the laser focus forms a reservoir, which serves to removeablated/irradiated material. If the reservoir is located at the back orside surface of the transparent target material, laser ablation and thewetting action of the reservoir will cause the formation of a void. Asthe focus of the laser is moved relative to the target, the void willform a channel. The advantage of these processes is that they allow theformation of higher aspect ratio channels (>100:1) with more complicatedthree-dimensional structure than is possible drilling blind holes. Inmuch the same way, the system's ability to monitor and actively controlthe laser and beam parameters enables the degree of irradiation (laserintensity at focus and laser “dose”) to be precisely defined as well asthe size/shape of the irradiated volume. Thus, as with waveguidewriting, it is possible to produce complicated three-dimensionalstructures, including tapered or periodic structures, using a“single-pass” exposure step which is not possible using current lasersand techniques. The ability to form such structures in a single pass isimportant, because it has been shown that material between “illuminatedtracks” is less easily removed than at the point of peak intensity.Therefore lines associated with multiple pass irradiations are stillclearly visible after prolonged etching, which can disrupt thesmoothness of the fluidic channels

The present invention enables more precise control over the irradiatedvolume and accumulated material exposure than is available using otherlaser irradiation methods, as a result of the integration of lasermonitors/controls and beam monitors/controls. It has been recently shownthat extremely small holes in the range of fifteen nanometers can bemachined utilizing ultrafast lasers of this type by A. Joglekar et al.,“A study of the Deterministic Character of Optical Damage by FemtosecondLaser Pulses and Applications to Nanomachining,” Appl. Phys. B. Theholes can be much smaller than the spot size since there is such adeterministic nature to ultrafast micro-machining. However, to get holesof equal size, the laser's pulse energy needs to be very preciselymaintained. A fiber-based chirped pulse amplification system assembledby telecomm compatible techniques allows much better pulse stabilitythan can be obtained by conventionally by regenerative amplifiers.Regenerative amplifiers typically have two percent average powerstability. With feedback circuits for power as described here and incommonly assigned and owned U.S. application Ser. No. 10/813,163,attorney docket no. A8698, filed on the same date as the instantapplication and also for the control of an external AOM, as described incommonly assigned and owned U.S. application Ser. No. 10/813,173,attorney docket no. A8699, filed on the same date as the instantapplication power can be made sufficiently constant such that repeatablefabrication of nanometer features may be possible. If lines are desiredof fifteen nanometers width, then the pulses must be delivered to thesample or the beam has to be scanned such that the separation betweenincident pulses is less than fifteen nanometers. Thus, for cm/secwriting speeds, repetition rates greater than 10 kilohertz are desired.This makes a fiber chirped pulse amplification system optimal for thisapplication.

This ultra-precise ablation process may be used for writing fine linesfor applications such as microcircuit production. Further reduction offeature size could be achieved by utilizing coherent affects such as aredescribed in U.S. Pat. No. 5,418,092. Ultrafast lasers have theadditional property that multiple pulses, which are not coherent, can beutilized and can overlap. Thus, coherent and incoherent affects can beutilized together. Finally, due to non-thermal ablation associated withfemtosecond laser processing it is possible to realize the theoreticalspatial limit for coherent effects.

These techniques can also be applied for depositing submicron wide linesa few monolayers thick on semiconductor substrates as is described inU.S. Pat. No. 6,656,539.

There are several emerging technologies, which rely upon the controlleddeposition of thin, often transparent, films, e.g., conductive layers ofITO, insulative layers of low-K dielectric and chemical resistancelayers of ZrO₂. Often, portions of such layers must be removed and/orinspected, while causing minimal damage to the underlying substrate. Insuch cases, the optimal choice of laser wavelength often depends uponthe nature of the substrate material. In the case of a thin target layer(or layers) on a transparent substrate, near IR femtosecond pulses maybe preferred since they can be precisely focused upon the target layerwithout interacting with the transparent substrate (whereas linearabsorption might be significant when using a UV source). In the case ofa thin transparent layer (or layers) on an opaque substrate, UVfemtosecond pulses may be preferred because of their high absorptioncoefficient (and correspondingly thin optical penetration depth) therebyconfining energy deposition to a thin layer at the surface.

Furthermore, this laser system (when equipped with diagnostics for LIBSand laser-induced fluorescence spectroscopy) would be well suited forthe characterization of a wide variety of materials based uponspectroscopic analysis of light generated during material ablation. Inparticular, spectroscopic data could be used to indicate that aparticular layer had been removed and to prevent further ablation.Furthermore, in an application requiring ablation of integratedcomponents composed of a variety of materials, this invention allows forthe combination of two or more harmonics (e.g., 1045 nanometers and 348nanometers). It may be desired to change the spot size, focus position,wavelength, pulse energy, pulse width and/or repetition rate quickly asthe laser ablation switches from material to material and layer tolayer. The laser system of the invention, allows for modulation of thepower distribution among the harmonics (via optical frequencyconversion), laser and beam control described as part of the enabledapplications above, and spectroscopic analysis.

Currently, excimer lasers are being used widely for annealing amorphousSi into a polycrystalline Si for thin film transistor (TFT) fabrication.The substrate is kept at a temperature of 300-400 degrees Celsius and alaser beam is scanned over a wide substrate to achieve annealing.Recently, it has been observed that if a femtosecond laser beam is usedfor scanning a substrate, a good quality film can be produced at a lowertemperature of 200 degrees Celsius. Hence, it is possible to use afemtosecond laser processing system as described above to achievequality Si films at a lower temperature in an industrial environment.

Laser pulse deposition techniques are used to deposit high quality filmswith precise control over various substrates. It has been observed thatuse of femtosecond laser pulses produces films with a better qualitythan a longer pulse width laser pulses. A laser processing system suchas one described above can enable deposition of various materials usingthe same system. For example, the proper control of the frequencyconversion module for example can allow optimization of laser parametersbased on target substrate optical properties.

Current techniques for laser welding of transparent materials involvethe deposition or placement of an absorbing material underneath atransparent material to allow for heat to be transferred from a lasersource (typically a long pulse near-IR system) to the weld zone. This isrequired since linear absorption is not available as a heat sourceduring laser propagation through transparent media. Femtosecond laserscan provide potential solution, since it is possible to couple heat fromthe laser to a transparent substrate via non-linear absorption. However,this technique has not been used in practice because the window betweenheating/melting and material ablation is very narrow, and thus requiresprecise control of laser processing parameters. The laser processingsystem described above is well suited for such an application because itincorporates a high repetition rate femtosecond laser source allowingfor the accumulation of heat from pulse-to-pulse. Furthermore, thisinvention allows for user control of laser processing parameters,particularly to control of the rate of heat deposition (through burstmachining, with variable repetition rate and pulse duration) andaccurate control of the position, size and shape of the focused beam.

The foregoing description of the preferred embodiments of the inventionhas been presented for purposes of illustration and description. It isnot intended to be exhaustive or to limit the invention to the preciseform disclosed, and modifications and variations are possible in lightof the above teachings or may be acquired from practice of theinvention. The embodiments were chosen and described in order to explainthe principles of the invention and its practical application to enableone skilled in the art to utilize the invention in various embodimentsand with various modifications as are suited to the particular usecontemplated. To this end, all patents, patent applications andpublications referred to herein are hereby incorporated by reference intheir entirety.

Thus, while only certain embodiments of the invention have beenspecifically described herein, it will be apparent that numerousmodifications may be made thereto without departing from the spirit andscope of the invention. Further, acronyms are used merely to enhance thereadability of the specification and claims. It should be noted thatthese acronyms are not intended to lessen the generality of the termsused and they should not be construed to restrict the scope of theclaims to the embodiments described therein.

1. An apparatus for impinging laser pulses on a target substrate, theapparatus comprising: laser means for generating bursts of compositepulses; control means that controls the laser means; and beammanipulation means for monitoring characteristics of the composite pulsebursts output from the laser means to generate feedback data for thecontrol means, and for manipulating the characteristics of the compositepulse bursts; and means for positioning the target substrate.
 2. Theapparatus as claimed in claim 1, wherein the beam manipulation meanscomprising: a power meter that measures the power of the laser pulsesoutput from the laser means; a photodiode that measures the repetitionrate of the laser pulses; and an optical gating device that measures thepulse duration of the laser pulses.
 3. The apparatus as claimed in claim1, wherein the beam manipulation means comprise means for opticallyconverting the fundamental frequency of a percentage of the generatedlaser pulses to one or more other optical frequencies.
 4. The apparatusas claimed in claim 3, the means for converting an optical frequencycomprising at least one optical member that converts a portion of thefundamental of the laser pulses into at least one higher order harmonicsignal.
 5. The apparatus as claimed in claim 4, wherein the opticalmember device comprises at least one non-linear crystal device.
 6. Theapparatus as claimed in claim 5, wherein the non-linear crystal devicefurther comprises a controller that controls the orientation of the atleast one non-linear crystal with respect to the input laser pulses. 7.The apparatus as claimed in claim 5, wherein the non-linear crystaldevice further comprises a dual-lens telescope that focuses the inputlaser pulses into the at least one non-linear crystal, wherein the atleast one non-linear crystal is disposed between the lenses of thetelescope.
 8. The apparatus as claimed in claim 7, wherein the dual-lenstelescope collimates the output of the at least one non-linear crystal.9. The apparatus as claimed in claim 4, wherein the means for convertingan optical frequency further comprise a spectrometer that measurespredetermined parameters of pulses output from the non-linear crystaldevice.
 10. The apparatus as claimed in claim 1, wherein the beammanipulation means comprises: a telescopic optical device to control thesize, shape, divergence or polarization of the laser pulses input intothe means for manipulating; and steering optics that control theimpingement location of the laser pulses on the target.
 11. Theapparatus as claimed in claim 10, the apparatus further comprising: abeam profiler that monitors characteristics of laser pulses output fromthe telescopic optical device; and a position detector that determines aposition of the laser pulses output from the steering optics.
 12. Theapparatus as claimed in claim 10, wherein the telescopic optical devicecomprises at least two cylindrical lens telescopes aligned along anoptical axis traversed by the laser pulses.
 13. The apparatus as claimedin claim 3, wherein the apparatus further comprises: means for directingthe fundamental and/or converted frequencies of the laser pulses and theimpingement location of the pulses with respect to the target substrate;means for focusing the fundamental and/or converted frequencies of thelaser pulses;
 14. The apparatus as claimed in claim 1, the means forpositioning comprising: a mounting surface for translating the targetwith respect to the laser pulses; and a motion control device coupled tothe mounting surface.
 15. The apparatus as claimed in claim 14, themeans for positioning further comprising an environmental chamber,wherein the mounting surface and the motion control device are disposedwithin the environmental chamber.
 16. The apparatus as claimed in claim1, wherein said apparatus is configured for modifying the refractiveindex of a target substrate.
 17. The apparatus as claimed in claim 1,wherein said apparatus is configured for surface marking, sub-surfacemarking and surface texturing of a target substrate.
 18. The apparatusas claimed in claim 1, wherein said apparatus is configured forfabricating holes, channels or vias in a target substrate.
 19. Theapparatus as claimed in claim 1, wherein said apparatus is configuredfor the deposition or removal of thin layers of material on a targetsubstrate.
 20. The apparatus as claimed in claim 1, wherein saidapparatus is configured for the joining, welding or fusing oftransparent materials.
 21. An apparatus as claimed in 1, wherein thelaser means comprises a fiber amplifier.
 22. An apparatus as claimed inclaim 21, wherein the laser means further comprise at least onestretcher and at least one grating compressor.
 23. An apparatus forimpinging pulses on a target substrate, the apparatus comprising: lasermeans for generating optical pulses, and for amplifying said pulses in asame fiber amplifier gain medium, wherein said laser means is configuredso that each of said pulses are incident on said same fiber laser gainmedium and emitted from the same fiber gain medium along a commonoptical path; control means that controls the laser means; and beammanipulation means for monitoring a characteristic of said opticalpulses, and for monitoring said target substrate, said beam manipulationmeans being operable to generate feedback data for the control meansbased on target and pulse information, the beam manipulating meanscomprising: a telescopic optical device to control the size, shape,divergence or polarization of the laser pulses input into the beammanipulation means, and steering optics that control an impingementlocation of the laser pulses on the target substrate; means forpositioning the target substrate.
 24. An apparatus for impinging pulseson a target substrate, the apparatus comprising: laser means forgenerating optical pulses; control means that controls the laser means;beam manipulation means for monitoring a characteristic of said opticalpulses, and for monitoring said target substrate, said beam manipulationmeans being operable to generate feedback data for the control meansbased on target and pulse information; and means for positioning thetarget substrate.
 25. The apparatus as claimed in claim 24, wherein saidlaser means comprises a fiber amplifier for amplifying said pulses in asame fiber amplifier gain medium, and wherein said laser means isconfigured so that each of said pulses are incident on said same fiberlaser gain medium and emitted from the same fiber gain medium along acommon optical path.
 26. The apparatus as claimed in claim 24, whereinthe beam manipulating means comprises: a telescopic optical device tocontrol the size, shape, divergence or polarization of the laser pulsesinput into the beam manipulation means, and steering optics that controlan impingement location of the laser pulses on the target substrate 27.The apparatus as claimed in claim 24, wherein said laser means producespulses having pulse widths in the range of femtoseconds to picoseconds.28. The apparatus as claimed in claim 24, wherein said apparatuscomprises one or more of a range finder, camera, and spectroscopic toolfor monitoring said target substrate.
 29. The apparatus as claimed inclaim 24, wherein said laser means comprises an amplified ultrashortfiber laser system producing output pulses in the range of about 10 fsto 1 ps.
 30. The apparatus as claimed in claim 24, wherein said pulsescomprise composite pulses forming a burst.
 31. The apparatus as claimedin claim 24, wherein a pulse characteristic being monitored comprises atleast one of time separation, pulse width, peak power, differentwavelength, and polarization.
 32. The apparatus as claimed in claim 24,wherein said beam manipulation means produces a beam at a fundamentalfrequency, at least one harmonic beam at a multiple of said fundamentalfrequency, and is configured to transmit information for diagnosticsfrom said beam manipulation means to said control means.
 33. Theapparatus as claimed in claim 24, wherein said apparatus is configuredfor modifying the refractive index of a target substrate.
 34. Theapparatus as claimed in claim 24, wherein said apparatus is configuredfor surface marking, sub-surface marking and surface texturing of atarget substrate.
 35. The apparatus as claimed in claim 24, wherein saidapparatus is configured for fabricating holes, channels or vias in atarget substrate.
 36. The apparatus as claimed in claim 35, wherein saidtarget substrate comprises a semiconductor substrate.
 37. The apparatusas claimed in claim 24, wherein said apparatus is configured for thedeposition or removal of thin layers of material on a target substrate.38. The apparatus as claimed in claim 24, wherein said apparatus isconfigured for the joining, welding or fusing of transparent materials.39. A method of laser-based device fabrication, comprising: irradiatinga target substrate with ultrashort laser pulses; monitoring a pulsecharacteristic of the ultrashort pulses, and a condition of the targetsubstrate, to obtain process information; modifying the pulsecharacteristic based the process information; and repeating the step ofirradiating, wherein said method of device fabrication comprises formingone or more of a hole, channel, trench, groove, and via in the targetsubstrate.
 40. The method as claimed in claim 39, wherein the ultrashortpulses comprise at least one pulse having a pulse width in the range ofabout 10 fs to 1 ps.
 41. The method as claimed in claim 39, wherein thedevice is configured for a microelectronic application, and the targetsubstrate comprises silicon.